Online entities Archives.com, FamilySearch.org and findmypast.com have announced a collaboration to create an index of the 1940 census when the census is released to the public in April 2012. This index will be online and free to the public. Archives.com had already announced they would be hosting the digital images of the 1940 census online for free access by the public.
This census indexing project has the support of the Federation of Genealogical Societies and the National Genealogical Society, among others. The full press release about this collaboration is available here.
More information on the project - including how you can join the indexing effort - is available at http://the1940census.com.